Closed-loop magnetoresistive current sensor system having active offset nulling

ABSTRACT

A magnetoresistive sensor system having resistive elements changing in ohmic value in the presence of a magnetic field of a current being measured. The variant values of the elements are amplified by some electronics that inherently add offset to the resultant values. The elements themselves also add an offset. The output of the electronics is modulated and then buffered as an output. This output is demodulated and integrated. The resultant signal is fed back to the input of the electronics to null out the offsets. The output of the buffer also goes to an inductive coil that is magnetically coupled to the resistive elements to null out the magnetic field from the current being measured. The buffer output indicates the magnitude of the current being measured. An oscillator outputs a signal to actuate the modulator and the demodulator. The oscillator signal also goes to a set/reset circuit for setting and resetting the resistive elements of the magnetoresistive sensor.

[0001] The invention pertains to current sensors and particularly tomagnetoresistive (MR) current sensors. More particularly, it pertains toMR sensors having low offsets.

SUMMARY OF THE INVENTION

[0002] The present current sensor is a closed-loop, non-contact,fast-responding, wide-bandwidth, field nulling current sensor. Thissensor reveals parameters equivalent to other closed-loop approachessuch as the closed loop Hall effect current sensor in the related art.It is the use of the MR sensor having a set/reset and offset nullingloop leads to ultra low offsets and ultra low offset drifts over allconditions in current measurement. It also has very low offset driftover wide ranges of operating temperatures. Because of the ultra lowoffset, the sensor provides very accurate measurements of current. Ithas an auto-zero offset reduction circuit, which has a separate feedbackpath to remove offset and offset drift from the circuit. The magneticsignal and offset signals are driven to their respective nulls byindependent feedback loops. The offset feedback loop is not used in Halleffect type current sensors. The transient noise and spikes caused bythe set/reset circuit are greatly reduced with this new feedback scheme.The sensor has a frequency response from DC to over 150 kHz and has aresponse time of less than one microsecond.

BRIEF DESCRIPTION OF THE DRAWING

[0003] FIG. l is an illustration of an MR closed loop current sensingsystem.

[0004]FIG. 2 is a functional diagram of the MR current sensing system.

[0005]FIG. 3 is a schematic of the MR current sensing system.

DESCRIPTION OF THE EMBODIMENT

[0006] An overview of a closed loop MR current sensor system 10 is shownin FIG. 1. Input primary current 12 can be in a conductor having one toN turns, where N can be any practical number. The magnetic flux 86 fromcurrent 12 of the primary current carrying conductor is concentrated bya high permeability core 26 and dropped across a gap 17 to produce amagnetic field 86. An inductive feedback or rebalance coil 25 produces amagnetic field 99 to oppose magnetic field 86. MR die 11 senses thedifference between magnetic fields 86 and 99. The output of MR die 11 isamplified, among other things, by an electronics loop 91, which includesan operational amplifier electronics 30, demodulator 31, bufferamplifier electronics 32, set/reset circuit 90, modulator 36 andintegrator 37. Electronics loop 91, along with coil 25, drives magneticfield 86 in gap 17 to a null value with a magnetic field 99. A path 97provides set/reset signals to MR die 11. Circuit 90 sets and resets orrotates the magnetic vector of MR die 11 180 degrees at a 1 kHz rate.This technique is used to produce an ultra low offset current sensor 10.Gap 17 is small relative to the core 26 cross-section and length toprovide better accuracy and improved shielding from stray externalmagnetic fields.

[0007] A conceptual layout of MR sensor system 10 is shown in FIG. 2.System 10 has an anisotropic MR Wheatstone bridge 11 next to aconductor(s) carrying a current 12 that generates a magnetic field 86.This magnetic field is typically concentrated across MR sensor 11through use of a magnetic core 26. In FIG. 3, MR sensor 11 is comprisedof four Permalloy (NiFe) resistors 13-16 that are connected end to end,and the magnetic field 86(B) of current 12 and 99 of rebalance coil 25changes the resistance of resistors 13, 14, 15 and 16. In the set/resetprocess, a high current (unrelated to the current being measured or thecurrent being fed back) pulse is sent through the set/reset strap by aMOSFET driver 78. This “sets” the magnetization of the Permalloy in onedirection such that a positive magnetic field (generated by theconductor or feedback coil or any other field source) will cause theresistance of two resistors to increase and the other two resistors todecrease. If a pulse of equal amplitude but opposite direction is thensent through the set/reset strap, this is a “reset” condition and eachresistor will change resistance opposite to the set condition. Theset/reset process and the magnetization of the Permalloy are independentof the conductor with current that is being measured. Resistors 13 and15 increase in resistance for a first direction of current 12 flow andresistors 14 and 16 decrease in the set condition. For the firstdirection of current 12 flow, resistors 13 and 15 decrease and resistors14 and 16 increase in resistance in the reset condition., The resistancechanges are proportional to the magnitude of the current 12 flow.Current 12 may have its magnitude measured in amperes and the magneticfield measured in Gauss. K_(B 17) is a transfer function inherent tosensor 11. Function 17 represents the effect of the magnetic field 86 ofcurrent 12 (and the field 99 generated inductive feedback coil 25) onthe resistors of sensor 11. Modulator or multiplier 18 of FIG. 2 iswhere the set/reset signal comes to sensor 11 from an oscillator 20. Thesignal is a one kHz square wave 21. The signal may be another kind ofsignal and/or have a different frequency. Summer 19 conceptuallyrepresents the superposition of a V_(off1) (MR offset voltage) to anoutput signal of sensor 11. K_(A) 23 represents the transfer functionfor a gap in core 26, which may be given in ampere-turns a device havingN₁ 24 turns in the first winding, which is the current 12 carrying wire.N₂ 25 represents the number of turns of the second winding (inductivefeedback coil), which couples the negative magnetic feedback (99) tosensor 11. The feedback coil has about 1000 turns. The magnetic coresuperposes the magnetic fields (99 and 86) from the feedback coil 25 andthe current 12, therefore it is represented by summer 26.

[0008] The output of sensor 11 goes to a summer 27 which adds an offsetV_(off2) 28 of the operational amplifier electronics 30 and the voltageoffset feedback to the output signal of sensor 11 to cancel the offsets.K_(C) 29 represents the transfer function, such as gain, of theoperational amplifier electronics 30. The output of electronics 30 goesto demodulator 31 to be demodulated. Items 46-49 are actually a part ofdemodulator 31. An input signal 21 from oscillator 20 sets thedemodulator 31 frequency. The demodulator sensor signal goes on to K_(D)32, which is the transfer function of the final operational or currentbuffer amplifier electronics 32 for sensor system 10. It has a largegain at DC and rolls off at above 1 kHz. The output goes to a transferfunction 33 of the inverse of the sum of the load resistance R_(L) 34 ofsystem 10 and the inductive impedance (R_(ind)+jωL_(ind)) of winding orcoil 25. Z_(ind)=(R_(ind)+j ω L_(ind)), where Z_(ind) is the impedanceof the inductive feedback coil which is the sum of the inductive coil'sresistance (R_(ind)) and reactance (j ω L_(ind)) where L_(ind) is theinductive coil's inductance. The output of the current sensor is givenin current and is typically run through a load (R_(L)) to measure avoltage out. An offset feedback signal from output 35 goes to amodulator 36. This offset feedback signal could also come from theoutput of 33 (tied after 32 on the schematic). Signal 21 from oscillator20 actuates demodulator 36. The de-modulated offset feedback signal goesto an integrator 37 that provides the integrated voltage offset feedbacksignal (DC) to summer 27 of operational amplifier electronics 30 withthe modulated signal level at about 1 kHz filtered.

[0009]FIG. 3 is a schematic of sensor system 10. Sensor 11 has theWheatstone bridge of resistors 13-16, connected end-to-end as describedabove. The bridge outputs go to the inverting and non-inverting inputs,respectively, via conductors 93 and 94 and 4.02K ohm resistors 39 and40, of differential amplifier 38 of operational amplifier electronics30. A 40.2 K ohm resistor 41 connects the non-inverting input to ground,and a 40.2 K feedback resistor 42 connects the output of amplifier 38 toits inverting input. The output of amplifier 38 goes to the invertinginput of amplifier 43 via a 4.02 K ohm resistor 44. The output ofamplifier 43 is connected to its inverting input via a 40.2 K ohmresistor 45. The non-inverting input of amplifier 43 is connected toground. A resistor to ground is not necessary here (for this op-amponly) since any offset because of mis-matched input resistance isremoved by the offset feedback. This removes one resistor. The outputgoes to the non-inverting of amplifier 47 of demodulator 31, via a 4.02K ohm resistor 46. The output is connected to the inverting input via a4.02 K ohm resistor 48. The non-inverting input is connected to groundvia a 2 K ohm resistor 49. Operational amplifiers 38, 43 and 47 areBA4560 amplifiers, although many other models could be used.

[0010] An input to a first terminal of a normally open switch 50 ofdemodulator 31 is connected to the output of amplifier 43. An input to afirst terminal of a normally closed switch 51 is connected to the outputof inverting amplifier 47. The output terminals of switches 50 and 51are connected to the inverting input of amplifier 52 of output bufferelectronics 32, via a 2 K ohm resistor 53. The actuators of switches 50and 51 of demodulator 31 are actuated with signal 21 from oscillator 20.Switches 50 and 51 of demodulator 31 are in an ADG413 quad SPST analogswitch device.

[0011] The non-inverting input of amplifier 52 is connected to groundvia a 2 K ohm resistor 54. The output of amplifier 52 is connected toits inverting input via a 56 picofarad capacitor 57. This connectionarrangement may be varied depending on the stability of the amplifier52. The output of amplifier 52 goes to the bases of NPN buffer outputtransistor 58 and PNP buffer output transistor 59. The collectors oftransistors 58 and 59 are connected to +15 volts and −15 volts,respectively. The emitters of transistors 58 and 59 are connectedtogether, and are connected to the output of amplifier 52 via a 316 ohmresistor 60. The common emitter connection of transistors 58 and 59 isthe output of buffer electronics 32. This output is connected to theinverting input of amplifier 52 via a 0.033 microfarad capacitor 55 anda 5.11 K ohm resistor 56 connected in series. A diode 61 has a cathodeand an anode connected to the collector and emitter, respectively, oftransistor 58. A diode 62 has a cathode and an anode connected to theemitter and collector, respectively, of transistor 59. Amplifier 52 is aBA4560 device; transistor 58 is a BCX55 device; and transistor 59 is aBCX52 device.

[0012] The output of first buffer electronics 32 is connected to a firstterminal of winding 25. Winding 25 provides the magnetic feedback to MRsensor 11 via a magnetic core 26 and gap 23. Core 26 and gap 23 arerepresented in FIG. 2 but not shown in FIG. 3. A second terminal ofwinding 25 is connected to the anode of diode 63 and the cathode ofdiode 64. The cathode of diode 63 is connected to the collector oftransistor 58. The anode of diode 64 is connected to the collector oftransistor 59. Diodes 61,. 62, 63 and 64 function as voltage clamps forcircuit protection.

[0013] The second terminal of winding 25 is the current output 65 ofsensor system 10. Output 65 is connected to ground via a 4,700 picofaradcapacitor. Output 65 is connected to one end of a 100 ohm RL resistor34. The other end of resistor 34 is connected to ground. This is theresistor used here, but the customer usually chooses this value. It canbe anything from zero to one megohm, although the device does notoperate over its full range of current with larger resistors. Voltageoutput 35 across resistor 34 is the indication of the amount of current12 flowing through the wire proximate to MR sensor 11. Also, output 65is connected as an offset feedback signal to the non-inverting input ofa buffer amplifier 66. Amplifier 66 is an LT1013 device. Although theLT1013's low offset and offset drift properties are not required, it isused as this buffer versus a cheaper amplifier because it is a spare onthe chip anyway.

[0014] The output of amplifier 66 is connected to its inverting inputand to input terminals of a normally open switch 67 and a normallyclosed switch 68. Switches 67 and 68 are of modulator 36. Actuators ofswitches 67 and 68 are actuated by signal 21 from oscillator 20. Theoutput terminals of switches 67 and 68 are connected to the invertingand non-inverting inputs of an amplifier 69 of integrator 37 via 24.9 Kohm resistors 70 and 71, respectively. Amplifier 69 is an LT1013 device.A 0.47 microfarad capacitor connects the output of amplifier 69 to itsinverting input. A 0.47 capacitor 73 connects the non-inverting input ofamplifier 69 to ground. The output of amplifier 69 is connected to asummer 27 point at the non-inverting input of amplifier 43, via 46.4 Kohm resistors 74 and 75. The inter-connection of resistors 74 and 75 isconnected to ground through a 0.47 microfarad capacitor 76. The signalfrom amplifier 69 to the summer 27 point is a voltage offset signal thatis used for nulling out offset signals (V_(off1)) 22 in MR sensor 11 andoffset signals (V_(off2)) of amplifier electronics 30.

[0015] Demodulator 31 and modulator 36 have normally open switches 50and 67, and normally closed switches 51 and 68. The switches, as notedabove, are paired off in demodulator 31 and modulator 36. In each pair,one switch is open and one switch is closed at any one given timeperiod. All of the four switches 50, 51, 67 and 68 are actuated by clockpulse 21. Since the sensed signal was modulated in MR sensor 11 at onekHz, demodulator 31 demodulates the signal back down to DC (andmodulates DC signals up to one kHz). Switches 67 and 68 take one kHzsquare wave 21 and alternately switch the incoming signal to thenon-inverting and the inverting inputs of operational amplifier 69. Thisin effect modulates any DC signal and demodulates a one kHz signal.Amplifier 69 then acts as an integrator to pass the DC signal asfeedback to operational amplifier electronics 30. Resistors 74 and 75and capacitor 76 also act to filter this voltage.

[0016] The set/reset mechanism 90 for MR sensor 11 is initiated by asignal 21 from oscillator 20. Oscillator 20 consists of an LM555 chip 77with a terminal 1 to ground. A terminal 2 is connected to thresholdterminal 6 that is connected to ground via a 0.01 microfarad capacitor80 and to VCC (+5 volts DC) via a 49.9 K ohm resistor 82. An outputterminal 3 provides about one kHz square wave output 21. Also, chip 77has a reset terminal 4 connected to the input of a high speed MOSFETdriver 78, and control terminal 5 connected to ground via a 0.01microfarad capacitor 79. Discharge terminal 7 is connected to terminal 6via a 21.5 k ohm resistor 81. VCC terminal 8 is connected to +5 voltsDC.

[0017] The output of set/reset driver 78 is connected to set/reset coilor strap 83 of MR sensor 11, via a 0.01 microfarad capacitor 84 and aneight ohm resistor 85 connected in series.

[0018] Associated with the power supply, there is a 5.1 volt Zener diode87 and 2 K resistor 88 tied between ground and +15 volts. This creates a5 volt supply for the switches 67, 68, 50 and 51 (through Vp) and device77. The approach creates a 5 volt supply, and improves performance overthe tying those pins to +15 volts. Basically, this configurationdecreases the “glitches” on the output each time the set or resetoccurs, for the price of two low-cost components.

[0019] Although the invention has been described with respect to aspecific preferred embodiment, many variations and modifications willbecome apparent to those skilled in the art upon reading the presentapplication. It is therefore the intention that the appended claims beinterpreted as broadly as possible in view of the prior art to includeall such variations and modifications.

1. A magnetoresistive current sensor system comprising: amagnetoresistive sensor having an output; operational amplifierelectronics having an input connected to the output of saidmagnetoresistive sensor, and having an output; a demodulator having aninput connected to the output of said operational amplifier electronics,and having an output; output buffer having an input connected to theoutput of said demodulator, and having an output; a coil, magneticallycoupled to said magnetoresistive sensor and connected to the output ofsaid output buffer; a modulator having an input connected to the outputof said output buffer; and an integrator having an input connected tothe output of said modulator and having an output connected to saidoperational amplifier electronics.
 2. The sensor system of claim 1,wherein: a first offset voltage is at the output of saidmagnetoresistive sensor; a second offset voltage and the first offsetvoltage are at the output of said operational amplifier electronics; andan offset voltage feedback signal from the output of said integratornulls out the first and second offset voltages.
 3. The sensor system ofclaim 2, wherein: a magnetic signal, having a magnitude, from said coilto said magnetoresistive sensor, magnetically nulls out a magnetic fieldsensed by said magnetoresistive sensor; and the magnitude of the coil'smagnetic signal is indicative of a magnitude of the magnetic fieldsensed by said magnetoresistive sensor.
 4. The sensor system of claim 3,further comprising: at least one conductor proximate to saidmagnetoresistive sensor; and wherein said at least one conductorconducts a current that radiates the magnetic field sensed by saidmagnetoresistive sensor.
 5. The sensor system of claim 4, wherein themagnitude of a signal at the output buffer is indicative of themagnitude of the current.
 6. The sensor system of claim 4, furthercomprising: a set/reset circuit connected to said magnetoresistivesensor; and an oscillator outputting a clock signal; and wherein: theclock signal goes to said set/reset circuit; said set/reset circuit setsand resets said magnetoresistive sensor; the clock signal goes to saiddemodulator to actuate said demodulator; and the clock signal goes tosaid modulator to actuate said modulator.
 7. The sensor system of claim4, wherein; said magnetoresistive sensor is a Wheatstone bridge ofresistors; and said resistors are Permalloy (NiFe) resistors.
 8. Amagnetoresistive sensor system comprising: a magnetoresistive sensor; aconductor proximate to said magnetoresistive sensor; a first amplifierconnected to said magnetoresistive sensor; a demodulator connected tosaid first amplifier; a second amplifier connected to said demodulator;a coil, coupled to said magnetoresistive sensor and connected to saidsecond amplifier; a modulator connected to said second amplifier; and anintegrator connected to said modulator and to said first amplifier. 9.The sensor system of claim 8, wherein said integrator outputs a firstsignal to null out an offset voltage, if any, of said magnetoresistivesensor.
 10. The sensor system of claim 9, wherein said integratoroutputs a second signal to null out an offset voltage, if any, of saidfirst amplifier.
 11. The sensor of claim 10, wherein: said coil couplesa magnetic signal to said MR sensor to null out a magnetic field of acurrent in said conductor, sensed by said magnetoresistive sensor; and amagnitude of the magnetic signal is indicative of a magnitude of thecurrent in said conductor.
 12. The sensor of claim 11, furthercomprising: a set/reset circuit connected to said magnetoresistivesensor; and an oscillator connected to said modulator, said demodulator,and said set/reset circuit.
 13. A magnetoresistive current sensor systemcomprising: a magnetoresistive sensor; an amplifier connected to saidmagnetoresistive sensor; a modulator connected to said amplifier; abuffer connected to said modulator; a modulator connected to saidbuffer; an integrator connected to said modulator and said amplifier;and an inductive device coupled to said magnetoresistive sensor andconnected to said buffer.
 14. The system of claim 13, furthercomprising: a set/reset circuit connected to said magnetoresistivesensor; and a signal generator connected to said modulator, demodulatorand set/reset circuit.
 15. A means for sensing current with offsetnulling, comprising: sensor means for magnetoresistively sensing amagnetic field; amplifier means for amplifying signals from said sensormeans; modulator means for modulating signals from said amplifier means;buffer means for buffering signals from said modulator means;demodulator means for demodulating signals from said buffer means; andintegrator means for integrating signals from said demodulator means,and adding integrated signals to said amplifier means.
 16. The means forsensing current of claim 15, wherein the integrated signals null outoffsets from said sensor means.
 17. The means for sensing current ofclaim 16, wherein the integrated signals null out offsets from saidamplifier means.
 18. The means for sensing current of claim 17, furthercomprising magnetic feedback means for providing signals from saidbuffer means to said sensor means to null out the magnetic field sensedby said sensing means.
 19. The means for sensing current of claim 18,further comprising a set/reset means for setting and resetting saidsensor means.
 20. The means for sensing current of claim 19, furthercomprising a generator means for generating a signal for said modulatormeans, demodulator means and set/reset means.